Saturday, May 29, 2010

DEFENCE MECHANISMS IN FISHES

INTRODUCTION

The Aquatic environment can be a dangerous place for a small fish. Fortunately, nature has bestowed a number of defense mechanisms on the fish, helping them to escape predators. From camouflage to poisons to flying, fish have a number of ways to find safety. They developed their own defense mechanisms against predators.

Fishes can hide from their predators, sometimes they run away, sometimes they burry themselves. These are the natural defence mechanism.

Camouflage is one of the most widespread defence modes used by substrate-dwelling animals, whereas transparency is generally found in open-water organisms. Both these defence types are regarded as effective against visually guided predators. We present here three assemblages of similarly-sized freshwater fish and shrimp species which apparently rely on camouflage and transparency to evade some of their potential predators. In one of the associations, there is a transition from cryptic colours and translucency to transparency of the component species according to the position each of them occupies in the habitat. The likeness between the fishes and the shrimps is here regarded as a type of protective association similar to numerical or social mimicry. Additionally, we suggest that the assemblage may contain Batesian-like mimicry components.

Mimicry When the subject of mimicry is raised, the first examples that come to mind are in the insect world. However a surprising number of mimics have been discovered among fresh water & marine fishes, the subject of this pictorial review. There is a need to provide a distinction between protective resemblance and mimicry In the former an animal resembles some object which is of no interest to its enemy, and in so doing is concealed; in the latter an animal resembles an object which is well known and avoided by its enemy, and in so doing becomes conspicuous. An animal resembles an object which is well known and is avoided or not preyed upon by its enemy, and in so doing becomes conspicuous. Mimicry applies only to animals those resemblance active animals. In addition to resemblance in color and morphology, the mimic may adopt a pattern of behavior to enhance the deception.

Animals both terrestrial and marine have been using biological warfare for centuries as a method of defense. It is my intent to familiarize the reader with the two main delivery methods used widely in the animal kingdom. The main delivery methods are stinging and secretion.

Some species live in a group is called school. As Prentice Stout notes in the Rhode Island Sea Grant Fact Sheet “Fish Schooling” roughly 80% of known fish use schools. One reason schooling is believed to work is because predators usually hunt for creatures smaller then they are. A school of small fish gives the illusion of a larger fish. The schooling fish are also playing their odds of not getting eaten as a predator cannot eat an endless amount of food. Fish in the center of the school are safer than those on the edges.

DEFENCE MECHANISMS

Natural Defence

The balloon fish has a pelagic, or open-ocean, life stage. Spawning occurs after males slowly push females to the water surface. The eggs are spherical and buoyant, floating in the water. Hatching occurs after roughly four days. The larvae are predominately yellow with scattered red spots. They are well developed with a functional mouth, eyes, and a swim bladder. Larvae less than ten days old are covered with a thin shell. After the first ten days, the shell is lost and the spines begin to develop. The larvae undergo a metamorphosis approximately three weeks after hatching. During this time, all the fins and fin rays are present and the teeth are formed. The red and yellow colors of the larvae do not persist into the juvenile phase and are replaced by the olives and browns; characteristic of adults. Dark spots also appear on the juvenile's underside. Pelagic juveniles are often associated with floating sargassum, and these spots may serve as camouflage from predators such as dolphin that swim below the seaweeds.

The puffer’s unique and distinctive natural defenses are a compensation for their particular form of locomotion. Puffers use a combination of pectoral, dorsal, anal, and caudal fins for propulsion that make them highly maneuverable but very slow, and therefore comparatively easy targets for predators. As a defense mechanism, puffers have the ability to inflate rapidly, filling their extremely elastic stomachs with water (or air when outside the water) until they are almost spherical in shape. Thus, a hungry predator stalking the puffers may suddenly find itself facing what seems to be a much larger fish and pause, giving the puffers an opportunity to retreat to safety. When lifted out of water there is a risk that puffers inflate with air. This may result in problems deflating again afterwards. When this happens with aquarium specimens the recommended course of action for fish keepers is to hold the puffer underwater by the tail, head upwards, and shake the fish gently until the air escapes out of the mouth.

Due to some unknown selection pressure, intronic and extra genic sequences have been drastically reduced within this family. As a result, they have the smallest-known genomes yet found amongst the vertebrate animals, while containing a genetic repertoire very similar to other fish and thus comparable to vertebrates generally. Since these genomes are relatively compact it is relatively fast and inexpensive to compile their complete sequences, as has been done for two species (Takifugu rubripes and etraodon nigroviridis).

Puffers are able to move their eyes independently, and many species can change the color or intensity of their patterns in response to environmental changes. In these respects they are somewhat similar to the terrestrial chameleon.

The turned-up nose of the Tetraodon suvattii gives it a pig nosed appearance and the black V-shaped mark on the back of its head looks distinctly like an arrowhead. Thus the common names of Pignose Puffer and Arrowhead Puffer, along with the name Mekong Puffer derived from its native origin. When they see the predator they burry themselves and leave their mouth and eye .After passes away the predator they moved from there.

Many fish use a coloration scheme called ‘countershading’ to hide from predators. These fish have light colored undersides and dark tops. As the National Aquarium in Baltimore notes in its online article “Fish Biology and Anatomy” many of these fish live in the open ocean. Seen from above these open ocean fish blend in with the dark depths below. Seen from below they blend in with the light filled waters and sky above. This is one useful defense mechanism fish use to avoid predators.

Camouflage :Camouflage and disruptive coloration are among the most widespread defence modes used by substrate-dwelling animals, whereas transparency is generally found in open-water organisms, both crypsis types being regarded as effective against visually guided predators. Different cryptic animals (e.g., insects and frogs) dwelling in the same environment or habitat usually have similar shapes and colours, a remarkable instance of convergence (Cott, 1940; Edmunds, 1974). Moreover, similarly coloured animals may form protective associations. In this defence type the associated animals are similar in colour, size, shape, and behaviour, which likely hamper the predator's ability to sort them out. For fishes the best known examples came from studies on reef fishes (e.g. Dafni & Diamant, 1984; Randall & McCosker, 1993; Krajewski et al., 2004), whereas records of such associations in freshwater seem to be lacking. We present here three instances of crypsis and association between freshwater fishes and shrimps which apparently rely on camouflaging colour pattern and transparency to evade some of their potential predators. Fig:Eleotris camouflaged colour pattern and become transparent

Mimicry :It may come as a surprise but those brightly colored coral reef fish are actually using camouflage to hide from and confuse predators. The bright colors of these fish help them blend in with the colorful corals around them. As the National Aquarium in Baltimore goes on to say, many fish use a defense technique called ‘disruptive coloration’ where patterns or lines help these fish blend in with their background. Fish such as the rockfish and frogfish are masters of camouflage, blending into rocks, corals, and plants. However these fish use camouflage to hide from prey, waiting to strike until prey are near. Also, many sea creatures including fish can change color for camouflage.

Nimbochromis is a small genus of haplochromine cichlids. They are known as sleeper cichlids or (in Chichewa) kaligono ("sleepers"), due to their unique behaviour. These piscivorous species are often seen lying motionless on the lake bottom near rocks. If predator gone approach, the Nimbochromis will "wake up" and try to seize them. Their coloration has an irregular dark cloudy pattern on ligher background; for one thing, this provides camouflage, but it is also suspected that it is – at least in some – evolving into aggressive mimicry by imitating a rotting fish carcass and thus luring scavengers to their dex.

A photo-graph of a snake eel was recently sent to the author from Tahiti to determine if it was a seasnake. It proved to be another snake eel Leiuranus semicinctusa less-likely mimic of a sea snake because the dark bars are broader than the pale interspaces and do not completely encircle the body

Hundreds of animals have evolved to look like other species order to fool predators into thinking they're more of a threat, or to sneak up on unsuspecting prey. In the Indo-Pacific lives a fish that does both and has the rare ability to switch between different disguises – the bluestriped fangblenny. In 2005, Isabelle Cote and Karen Cheney from the University of Queensland discovered that a small reef Alectis ciliaris fish called the The juvenile of the carangid fish has long been presumed to be a mimic of Alectis ciliaris bluestriped fangblenny (Plagiotremus rhinorhynchos) is also a dynamic mimic. Its model is the bluestreak cleaner wrasse Labroides dimidiatus, an industrious species that provides a cleaning service for other reef visitors by picking off parasites and mucus from hard-to-reach places. The fangblenny's intentions are less welcome. Its resemblance to the helpful wrasse allows it to get close enough to mount quick attacks on larger fish, biting off scales and skin.

Cote and Cheney found that fangblennies have two guises. In one, it has a black body and an electric blue stripe that mimics the wrasse, but in the other, it's body is a very different brown, olive or orange with white or light-blue/green stripes. The fish can change from one to the other at will, and uses the non-mimicking colours to blend in with shoals of other fish.

Now, Cheney has provided further evidence for the opportunistic colour changes of this con artist. She captured 34 fangblennies of various colours and after 60 minutes alone, all the mimics had switched to non-mimic colours - it seems that there's no point putting on a disguise if there's no one around to see it.

A disguise may look right to us, but our colour vision is very different to that of most animals, including those whose reaction actually matters. To get a more objective view of the fangblenny's disguise, Cheney analysed the light reflecting off its scales when it went through its different colour phases. Sure enough, its black-and-blue form reflected light in almost exactly the same way as a real cleaner wrasse would.

The fangblenny's other colours also proved to be a match to other reef fish. The olive forms were most likely to be found among blue-green chromis, the brown forms mostly swam with the brown and white-coloured two-tone wrasse and the orange forms associated with orange Lyretail Anthias. In each of these cases, the pattern of light reflected off the fangblenny's coat matched that of its preferred companion.

The bluestripe fangblenny's many faces gives it great versatility. By matching the colours of a variety of different fish, it greatly expands the area of reef where it can safely hide from both predators and potential victims. Unlike the mimic octopus, it makes no effort to change its body shape and some of its models, like the chromis, are very different. But in a shoal, that hardly matters. A superficial resemblance to the surrounding throng may be advantage enough.

A fake eyespot near the tail can confuse predators by drawing attention from the real eye says the National Aquarium in Baltimore. As the confused predator goes after the fake eyespot the preyed upon fish can swim away and escape.

Fish That Jump Away From Predators: Tilapia for 2 meters to escape from predators. Tilapia has many predators from which it jump away from. Tilapia rapidly beats its tail fin to launch out of the water to glide through the air.

Fish That Fly Away From Predators:The flying fish can glide for 200 meters (655 feet) over the water reaching a height of 1.2 meters (4 feet) to escape from predators as the National Geographic notes in its online “Flying Fish” profile. The flying fish has many predators including tuna, mackerel, and swordfish from which it flies away from. The flying fish rapidly beats its tail fin to launch out of the water and then spreads its pectoral fins to glide through the air.

Spines Deter Fish Predators:Some fish are more aggressive with their defense against predators. Rather than run or hide these fish protect themselves with spines. The pufferfish, known for inflating itself against predators also often has spines. “Ocean Surgeonfish (Acanthurus bahainus)” that the surgeonfish has a sharp spine at the base of its tail that can do harsh damage to a would be predator.

A harmless, very active and sociable fish, clown loaches are best kept in groups of 3-4 or more. Due to their potentially large size.

These fish have bifurcated subocular (located under the eyes) spines, which are used as a defence mechanism and for obtaining prey. If a loach deploys its spines while caught in a net, untangling it is difficult and can result in injury to the handler or the fish. Aquarists recommend that large specimens are double or triple bagged, or placed in a solid container when being moved.

When kept in groups smaller than 5, clown loaches may spend lots of time hiding under obstacles in the water. Tiger barbs and Panda corydoras associate happily with clown loaches, and the three fish may school together.

Clown loaches make clicking noises when they are excited or during feeding. This sound is produced by the grinding of their pharyngeal teeth. Sometimes clown loaches swim on their sides, or upside down, and appear ill, or lie on their sides on the bottom of the tank and appear to be dead. This is normal behavior.
The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii which also, however, possess lepidotrichia. These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles).

Chemical Defence: Animals both terrestrial and marine have been using biological warfare for centuries as a method of defense. It is my intent to familiarize the reader with the two main delivery methods used widely in the animal kingdom. My research revolves around two terrestrial and three marine species. The main delivery methods are stinging and secretion.

In the marine environment I would like to explain these delivery methods through the use of three different species. The first of these will be the Volitan Lion Fish. This particular species is a part of the scorpion family of fish. As with all marine fish that are part of the phylum Pterois, the delivery methods and appearance of the animals are similar. As you can guess from the earlier information, these Lionfish deliver their toxin through the spiny tips on the dorsal and pectoral fins much like the scorpion and his singular spine at the tip of his tail. Each of the spines of the truly gorgeous fish can deliver a potent toxin, which affects the nervous system of its prospective prey, in the event of hunting for food, or as a defense method when attacked. This species is not very aggressive either in the wild or in a home aquarium though it is territorial in nature. The belief is that the toxin is developed based upon the diet of the fish. Meaning that it does not intentionally develop its own toxin as a part of its genetic structure but rather as an adaptation for its defense. Though specifically what items it feeds on to produce this toxin is not clear, there is precedence indicating this as the source.

Another marine animal, which delivers a toxin for defense purposes, is the Boxfish, also known as the "Neutron Bomb" fish. As its name indicates this animal can wipe out anything in close proximity to it when it becomes frightened. The toxin this fish releases is even toxic to itself. To protect itself from its own toxin this fish releases the toxin as it quickly leaves the area. The delivery method for this fish is excretory through its pores.

Tetraodontidae is a family of primarily marine and estuarine fish. The family includes many familiar species which are variously called puffers, balloonfish, blowfish, bubblefish, globefish, swellfish, toadfish, toadies, honey toads, and sea squab.They are morphologically similar to the closely related porcupinefish, which have large conspicuous spines (unlike the small, almost sandpaper-like spines of Tetraodontidae). The scientific name, Tetraodontidae, refers to the four large teeth, fused into an upper and lower plate, which are used for crushing the shells of crustaceans and mollusks, and red worms, their natural prey. The Puffer Fish species includes about 100 different species and is probably the best known of the toxic fish. As with most toxic fish it prefers warm tropical waters and is small in comparison to non-toxic fish species. The puffer is somewhat more difficult to deal with from a human perspective, as there is no antidote available for its toxin. The puffer both excretes, and in some species stings its predators with spines that protrude from its body. The toxin contained in this fish is tetrodoxin, which paralyzes its prey. Neurotoxins seem to be the preferred defense weapon of choice. It has been determined that this toxin is "about a thousand times deadlier than cyanide".

Puffer fish are the second most poisonous vertebrate in the world, the first being a Golden Poison Frog. The skin and certain internal organs of many tetraodontidae are highly toxic to humans, but nevertheless the meat of some species is considered a delicacy in both Japan (as fugu) and Korea (as bok). If one is caught while fishing, it is recommended that thick gloves be worn to avoid poisoning and getting bitten when removing the hook.

This neurotoxin is found primarily in the ovaries and liver, although smaller amounts exist in the intestines and skin, as well as trace amounts in muscle tissue and in its blood. The poison is made by bacteria of the genus vibrio and may actually enter the fish by consuming prey that does possess the poison already.

The tetraodontidae contains at least 121 species of puffers in 19 genera.They are most diverse in the tropics and relatively uncommon in the temperate zone and completely absent from cold waters. They are typically small to medium in size, although a few species can reach lengths of 100 centimetres (39 in).

By Electric organs:Some rays have electric organs, organic `batteries' formed from modified muscle tissue. The cells of these organs form tiny stacks of batteries in series to increase the voltage, while adjacent stacks produce a parallel effect to increase the amperage or current flow. Skates have elongated, spindle-shaped electric organs in their tails, which may serve in self-defense to ward off potential predators. Electric rays have large, kidney-shaped electric organs in their pectoral disks, which may be primarily defensive in small-mouthed species that eat tiny bottom invertebrates. Some large electric rays also use these organs offensively, to stun pelagic prey.

These flattened, bottom-dwelling and pelagic rays have rounded expanded pectoral fins forming a broad thick disk with the head and body, and a short stout tail with a broad caudal fin and 2, 1, or no dorsal fins. All have a pair of powerful, kidney-shaped electric organs at the bases of the pectoral fins. Large individuals are capable of delivering a sudden shock of up to 220 volts. Electric rays use their `batteries' to defend themselves and to stun their prey. All are live-bearers. 4 families and 43 species, of which 2, the Torpedinidae and Narkidae, and at least 5 species occur in the area.

By fin & jaw:Thresher sharks use their long tails to stun predator. Before striking, the sharks compact of predator by swimming around them and splashing the water with its tail, often in pairs or small groups. Threshers swim in circles to drive predators into a compact mass, before striking them sharply with the upper lobe of its tail to stun them.

Spinner sharks charge vertically through the predator, spinning on their axis with their mouths open and snapping all around. The shark's momentum at the end of these spiralling runs often carries it into the air.

Predator avoidance by schooling:There are a number of reasons fish travel in schools and one is for safety from predators. As Prentice Stout notes in the Rhode Island Sea Grant Fact Sheet “Fish Schooling” roughly 80% of known fish use schools. One reason schooling is believed to work is because predators usually hunt for creatures smaller then they are. A school of small fish gives the illusion of a larger fish. The schooling fish are also playing their odds of not getting eaten as a predator cannot eat an endless amount of food. Fish in the center of the school are safer than those on the edges.

Forage fish are small fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish are small, filter feeding fish such as herring, anchovies and menhaden. Forage fish compensate for their small size by forming schools. Some swim in synchronised grids with their mouths open so they can efficiently filter feed on plankton.[8] These schools can become huge, moving along coastlines and migrating across open oceans.

The shoals are concentrated fuel resources for the great marine predators.

These sometimes immense gatherings fuel the ocean food web. Most forage fish are pelagic fish, which means they form their schools in open water, and not on or near the bottom (demersal fish). Forage fish are short-lived, and go mostly unnoticed by humans, apart from an occasional support role in a documentary about a great ocean predator. The predators are keenly focused on the shoals, acutely aware of their numbers and whereabouts, and make migrations themselves, often in schools of their own, that can span thousands of miles to connect with, or stay connected with them.It is commonly observed that schooling fish are particularly in danger of being eaten if they are separated from the school.Several anti-predator functions of fish schools have been proposed.

Confusion effect – One potential method by which fish schools may thwart predators is the ‘predator confusion effect’ proposed and demonstrated by Milinksi and Heller (1978).This theory is based on the idea that it becomes difficult for predators to pick out individual prey from groups because the many moving targets create a sensory overload of the predator's visual channel. "Shoaling fish are the same size and silvery, so it is difficult for a visually oriented predator to pick an individual out of a mass of twisting, flashing fish and then have enough time to grab its prey before it disappears into the shoal."

Many eyes effect – A second potential anti-predator effect of animal aggregations is the ‘many eyes’ hypothesis. This theory states that as the size of the group increases, the task of scanning the environment for predators can be spread out over many individuals. Not only does this mass collaboration presumably provide a higher level of vigilance, it could also allow more time for individual feeding.

Dilution effect – A third hypothesis for an anti-predatory effect of fish schools is the ‘encounter dilution’ effect. The dilution effect is an elaboration of safety in numbers, and interacts with the confusion effect. A given predator attack will eat a smaller proportion of a large shoal than a small shoal.Hamilton proposed that animals aggregate because of a “selfish” avoidance of a predator and was thus a form of cover-seeking.Another formulation of the theory was given by Turner and Pitcher and was viewed as a combination of detection and attack probabilities.In the detection component of the theory, it was suggested that potential prey might benefit by living together since a predator is less likely to chance upon a single group than a scattered distribution. In the attack component, it was thought that an attacking predator is less likely to eat a particular fish when a greater number of fish are present. In sum, a fish has an advantage if it is in the larger of two groups, assuming that the probability of detection and attack does not increase disproportionately with the size of the group.

Schooling forage fish are subject to constant attacks by predators. An example is the attacks that take place during the African sardine run. The African sardine run is a spectacular migration by millions of silvery sardines along the southern coastline of Africa. In terms of biomass, the sardine run could rival East Africa's great wildebeest migration.

Sardines have a short life-cycle, living only two or three years. Adult sardines, about two years old, mass on the Agulhas Bank where they spawn during spring and summer, releasing tens of thousands of eggs into the water. The adult sardines then make their way in hundreds of shoals towards the sub-tropical waters of the Indian Ocean. A larger shoal might be 7 kilometres (4 mi) long, 1.5 kilometres (1 mi) wide and 30 meters (100 ft) deep. Huge numbers of sharks, dolphins, tuna, sailfish, Cape fur seals and even killer whales congregate and follow the shoals, creating a feeding frenzy along the coastline.

When threatened, sardines instinctively group together and create massive "bait balls". Bait balls can be up to 20 meters (70 ft) in diameter. They are short lived, seldom lasting longer than 20 minutes.

It is difficult to observe and describe the three dimensional structure of real world fish shoals because of the large number of fish involved. Techniques include the use of recent advances in fisheries acoustics.

Parameters defining a fish shoal include:

Density – The density of a fish shoal is the number of fish divided by the volume occupied by the shoal. Density is not necessarily a constant throughout the group. Fish in schools typically have a density of about one fish per cube of body length.

Polarity – The group polarity describes the extent to which the fish are all pointing in the same direction. In order to determine this parameter, the average orientation of all animals in the group is determined. For each animal, the angular difference between its orientation and the group orientation is then found. The group polarity is the average of these differences (Viscido 2004).

Nearest neighbour distance – The nearest neighbour distance (NND) describes the distance between the centroid of one fish (the focal fish) and the centroid of the fish nearest to the focal fish. This parameter can be found for each fish in an aggregation and then averaged. Care must be taken to account for the fish located at the edge of an fish aggregation, since these fish have no neighbour in one direction. The NND is also related to the packing density. For schooling fish the NND is usually between one-half and one body length.

Nearest neighbour position – In a polar coordinate system, the nearest neighbour position describes the angle and distance of the nearest neighbour to a focal fish.

Packing fraction – The packing fraction is a parameter borrowed from physics to define the organization (or state i.e. solid, liquid, or gas) of 3D fish groups. It is an alternative measure to density. In this parameter, the aggregation is idealized as an ensemble of solid spheres, with each fish at the center of a sphere. The packing fraction is defined as the ratio of the total volume occupied by all individual spheres divided by the global volume of the aggregation (Cavagna 2008). Values range from zero to one, where a small packing fraction represents a dilute system like a gas.

Integrated conditional density – This parameter measures the density at various length scales and therefore describes the homogeneity of density throughout an animal group.

Pair distribution function – This parameter is usually used in physics to characterize the degree of spatial order in a system of particles. It also describes the density, but this measure describes the density at a distance away from a given point. I found that flocks of starlings exhibited more structure than a gas but less than a liquid.

Fish schools are faced with decisions they must make if they are to remain together. For example, a decision might be which direction to swim when confronted by a predator, which areas to stop and forage, or when and where to migrate. How are these decisions made? Do more experienced 'leaders' exert more influence than other group members, or does the group make a decision by consensus? A recent investigation showed that small groups of fish used consensus decision-making when deciding which fish model to follow. The fish did this by a simple quorum rule such that individuals watched the decisions of others before making their own decisions. This technique generally resulted in the 'correct' decision but occasionally cascaded into the 'incorrect' decision. In addition, as the group size increased, the fish made more accurate decisions in following the more attractive fish model.Consensus decision-making, a form of collective intelligence, thus effectively uses information from multiple sources to generally reach the correct conclusion.

Function of Senses and brain in defence mechanism: Cartilaginous fishes have well-developed sense organs. Their eyes are usually large and well-developed, particularly in many deep-sea sharks. A few deep-water electric rays have degenerate eyes and are blind. Some sharks have a binocular field of vision, but the eyes of most cartilaginous fishes have virtually independent fields. Many cartilaginous fishes have vision adapted for low light, nocturnal activity or deep-water conditions, and poor color definition, and have retinas densely packed with rods and few cones. Some day-active sharks, including the Great white shark (Carcharodon carcharias), have numerous cones as well as rods, and may have good color vision and high visual acuity. Many deepwater cartilaginous fishes have a prominent layer of reflective material (the tapetum lucidum) behind their retinas, which serves to reflect light passing into the eye back into the retina, and hence increases its sensitivity. The eyes of these species glow bright green or yellowish when caught, from reflected light. The sense of smell (olfaction) is well-developed in cartilaginous fishes, which have large nostrils and olfactory organs. Some sharks can detect attractive substances at over one part per million parts of sea water, and are able to follow scent trails directionally and from great distances. Such sharks will swim against a current, tracking the scent trail to its source. Olfactory cues may play some role in orientation of cartilaginous fishes, perhaps in finding receptive mates or other members of their species, or locating specific areas, but this needs further research. Taste buds are well-represented in the mouths of cartilaginous fishes; some sharks are very selective on potential food items, swallowing or disgorging them after taking them into their mouths. The inner ears of cartilaginous fishes have large semicircular canals for maintaining equilibrium, but their sound-detecting apparatus is of limited range and complexity compared to birds and mammals. Sharks respond best to the lower sound frequencies, below 1000 hz, corresponding to many natural underwater sounds. Some low sound frequencies are attractive to sharks and may draw them from considerable distances. No cartilaginous fishes are known to produce underwater sounds, unlike many bony fishes.

Related to sound reception is the lateral line canal system of cartilaginous fishes and other aquatic vertebrates, a network of tubes below the skin, with several branches on the head but with a single line, the lateral line proper, running along the body on both sides and extending to the caudal fin. These canals have short tubes with external pores opening at frequent intervals to the outside, and have sensory cells that are responsive to low-frequency, close-range water vibrations. These can aid in avoidance of obstacles, location of prey, and detection of low-frequency sound. A similar function has been suggested for the pit organs, blind subdermal pockets with similar sensory cells as in lateral line canals, and with pores connecting to the surface. Pit organs are scattered along the body and may be very numerous in some large sharks such as hammerheads.

Cartilaginous fishes are equipped with the ampullae of Lorenzini, clusters of elongated, blind tubes with sensory cells on the blind end, a gelatinous filling, and openings to the outside; these form conspicuous groups of pores on the head and snout. These ampullae are sensitive to electrical fields, and provide a means for cartilaginous fishes to locate potential prey or one another by sensing the electrical fields produced by muscles and nerves. The ampullae may also function in navigation; cartilaginous fishes, moving through the water, produce electric fields that vary directionally according to the position of the cartilaginous fish relative to the Earth's electromagnetic field. These localized electrical fields can be detected by the ampullae and may provide directional information for long-distance navigation without visual, olfactory, or other sensory cues.

The input of the various sensory organs of cartilaginous fishes are integrated by their brains, and provide the individual a multifaceted `picture' of its environment. Pioneer researchers on sense organs, preoccupied with shark attack and ways to prevent it by studying how shark sense organs worked, concentrated on each sense organ independently rather than looking at all of them as an integrated unit. As with most living organisms, cartilaginous fishes do not rely on a single sense to interpret their environment but use all their senses to locate prey, other members of their species, or enemies. Cartilaginous fishes have fairly simple brains compared to those of large, advanced mammals, with the forebrain usually not greatly enlarged. Surprisingly these fishes have large brains proportional to their body size, and overlap birds and more primitive mammals in their ratios of brainweight to bodyweight. The devilrays (Family Mobulidae) may have the largest brains of any cartilaginous fishes.

Immune Defense Mechanisms in Fish to Protozoan and Helminth Infections:Fish respond to parasite infections (and infestations) by the production of antigen specific IgM-like antibodies as well as by the elaboration of nonspecific soluble factors and phagocytic cells. Fish infected with the hemoflagellates Trypanosoma and Cryptobia generally elicit antibody and complement dependent responses. The levels of these responses vary depending on ambient temperature fluctuations. Below 10–15°C there is an almost complete depression of immune responsiveness. The protozoan that has received the greatest emphasis regarding studies of immunity is Ichthyophthinus multifihis. Both primary and secondary antibody responses are produced in fish to this parasite. Cellular responses are also produced against "Ich." These cells (nonspecific cytotoxic cells) may provide an important (but previously not described) component of anti-parasite resistance.The second major group of parasites considered in this review are categorized as helminths. Among these, the cestodes, trematodes (mono- and digenetic), and Acanthocephala have been studied for elicitation of immune responses in fishes. For virtually all organisms studied, the host response was mediated via antibodies (plus complement in most cases). Cellular responses (neither antigen specific nor phagocytic activities) have not been shown to mediate any type of anti-helminth response in fishes.

DISCUSSION

Predation risk is recognized as a major selective force in evolution of numerous traits developed by many organisms to reduce their susceptibility to predation (Sih 1987; Abrams 2001). These traits include antipredator behavior such as reduced activity and increased use of shelters to decrease detection, or chemical and morphological defences to diminish the probability of a successful capture. Susceptibility to predation of exotic prey species introduced to novel areas is regarded as a potentially important proximate factor in their ability to become established(Lodge 1993), as natural predators contribute to biotic resistance by killingand eating exotic species. If natural enemies such as predators and parasites are rare or have little impact against new species, invaders may have an important advantage, which couldbe essential to their success. Natural enemies also drive community invasibility by altering competitive interactions between native and exotic prey species. This could be either through enemy-mediated apparent competition(Chaneton & Bonsall 2000), or when predators reduce the abundance of their prey, allowing potential competitors to coexist .Understanding the mechanisms that influence prey behaviours such ceptibility within new predator prey interactions emerging after a recent colonization may be important for understanding the success of the invasion. For example, immobility, decreased activity and use of shelters between interactions between stones of bottom sediments, as well as morphological structures such as spines or carbonate deposits, have been reported against predation (Andersson et al. 1986;).